1. Technical Field
The present invention relates to polyglycerol monoethers and processes for producing the same. More specifically, it relates to polyglycerol monoethers of high quality, and processes for producing the same. Such polyglycerol monoethers are useful in areas of, for example, pharmaceutical drugs, foodstuffs, cosmetics and biochemistry.
2. Background Art
Polyglycerol monoethers such as polyglycerol monoalkyl ethers have been produced according to various processes. The processes include, for example, a process (1) in which 1 mole of epichlorohydrin is added to an alcohol, dehydrochlorination and ring-closing are carried out under basic conditions, and ring-opening is carried out with a diluted sulfuric acid, and these procedures are repeated until a target degree of polymerization is obtained. Polyglycerol monoethers obtained by this process, however, may be not suitable for use typically in foodstuffs, cosmetics, detergents or cleaning agents, because they may contain chlorine compounds and there remains some doubt about their safety. In addition, the process invites complicated reaction steps and high cost in order to produce a product having a high degree of polymerization, because a product having a single degree of polymerization alone is obtained according to the process.
Another process is a process (2) of adding glycidol to and polymerizing with an aliphatic alcohol (comparative examples of Japanese Unexamined Patent Application Publication (JP-A) No. Hei 9-188755). According to the process (2), glycidol can be surely added to a compound having a phenolic hydroxyl group, such as an alkylphenol, because of high reactivity of hydroxyl group. However, when glycidol and an aliphatic alcohol are subjected to reaction according to a regular procedure of this process, self-addition polymerization occurs, and this invites by-production of large quantities of impurities including unreacted aliphatic alcohol and polyglycerols. This is because the hydroxyl group in an aliphatic alcohol has low reactivity, and a glycidol molecule is added to a hydroxyl group of another glycidol molecule in the self-addition polymerization.
Yet another process is a process (3) of adding a hydroxyl-protected glycidol to an aliphatic alcohol and carrying out deprotection. Japanese Unexamined Patent Application Publication (JP-A) No. Hei 9-188755, for example, discloses a production process of reacting an aliphatic alcohol with a glycidyl ester, such as glycidyl acetate, in the presence of an alkali metal catalyst. Japanese Unexamined Patent Application Publication (JP-A) No. Hei 9-235246 discloses a production process of reacting an organic hydroxyl compound with benzyl glycidyl ether in the presence of a phase-transfer catalyst. These processes (3), however, carry out protection and deprotection of glycidol and are thereby complicated in procedures, although target compounds are obtained. In addition, these processes are not chemically and industrially safe when an acid hydride, for example, is used as a protecting reagent, because an acid formed in the system may invite an abnormal reaction.
Japanese Unexamined Patent Application Publication (JP-A) No. Sho 56-20534 discloses a process (4) of adding glycidol to an aliphatic alcohol in a nonpolar solvent such as xylene. According to the process (4), however, some doubt remains about the safety of a residual solvent in the resulting product, and the product may not be suitable for use in foodstuffs and cosmetics. In addition, this process is poor in productivity.
Japanese Unexamined Patent Application Publication (JP-A) No. Hei 6-293688 discloses a process (5) of reacting a polyhydroxy compound with an alkyl sulfate to yield a polyhydroxymonoalkyl ether. According to the process (5), however, there remains a considerable amount of unalkylated polyglycerols, and the polyglycerol monoalkyl ether may not sufficiently exhibit its original detergency. In addition to a monoalkyl compound, there is by-produced a considerable amount of a dialkyl compound, and this causes high crystallinity and low solubility in water.
Japanese Unexamined Patent Application Publication (JP-A) No. Sho 57-197235 discloses a process (6) of reacting an alkyl glycidyl ether with acetal or ketal of glycerol to synthetically form a 4-(2′-hydroxy-3′-alkoxy)propoxymethyl-1,3-dioxolane, and hydrolyzing this to thereby yield a 2-hydroxy-3-alkoxypropyl glyceryl ether. The resulting polyglycerol monoalkyl ether prepared according to the process (6), however, is expensive, and it is difficult to use the product as raw materials typically for foodstuffs.
As is described above, polyglycerol monoethers are known substances but have not been generally used, because they contain large amounts of impurities such as polyglycerols and dialkyl components and are high in production cost.
Analogous compounds which have been industrially used include polyglycerol mono(fatty acid) esters and polyoxyethylene monoalkyl ethers, and they are used in a variety of articles such as cosmetics and detergents or cleaning agents based on their activities. However, polyglycerol fatty acid esters are limited in use, because they are insufficient in resistance to hydrolysis, to salts, and to acids. Polyoxyethylene monoalkyl ethers may yield formaldehyde and are low in water-solubility. Such formaldehyde has been perceived as a problem. Accordingly, demands of the market have been made on replacements of these compounds.
Patent Document 1: Japanese Unexamined Patent Application Publication (JP-A) No. Hei 9-188755
Patent Document 2: Japanese Unexamined Patent Application. Publication (JP-A) No. Hei 9-235246
Patent Document 3: Japanese Unexamined Patent Application Publication (JP-A) No. Sho 56-20534
Patent Document 4: Japanese Unexamined Patent Application Publication (JP-A) No. Hei 6-293688
Patent Document 5: Japanese Unexamined Patent Application Publication (JP-A) No. Sho 57-197235